(i) Field of the Invention
This invention relates generally to pill resistant polyester fabrics. More Particularly, it relates to an improved finishing process for reducing the pilling propensity of certain fabrics containing polyester staple fibers, which types of fabric are normally highly prone to pill (e.g., on account of the filaments having high tenacity and low denier per filament; or the fabric having loose weave or spaced loops, or employing single yarns). More specifically still, the field of the invention encompasses the spraying of fine particles of alkaline saponification agents onto the fabric's surface and thereafter heating the sprayed fabric.
(ii) Prior Art
The problem of "pilling " has been known to the textile industry for a long time. By it is understood the formation of small nodules as a result of rubbing or abraiding woven or knitted fabrics. Textile materials which are strongly prone to pilling often become unsightly even after short use.
Generally, two stages are involved in pilling:
(a) The gradual working up of fibers onto the surface of the material to produce a nap which then; PA0 (b) Rolls together, tangles and felts to form nodules.
With wool or cellulosic fibers, these nodules are soon rubbed off during use because of the comparatively low strength of these fibers. However, the situation is more serious with the high-strength synthetic staple fibers, including standard polyester fibers. The reasons are as follows:
Their smooth surface and round cross-section favor the fibers working out of the material. The great tensile and flexing strengths, and also the abrasion resistance, of the polyester fibers are desirable from the point of view of good durability in wear, but they prevent a rapid tearing off, breaking or rubbing off of the nodules.
Materials containing only polyester staple fibers have a greater tendency to pilling than those mixtures of these fibers with either natural or regenerated fibers. With fabrics made of continuous filament polyester, the conditions for the formation of nodules are absent, since there are no fiber ends to protrude.
The textile manufacturer can often reduce the tendency to pilling by an appropriate selection of fibers and by taking special precautions during the manufacture of the yarn or fabric, for instance, by using polyester fiber having a long staple length; using a coarse count; imparting a pronounced crimp; employing a high twist in the yarn (or in doubling); employing doubled yarns instead of single yarns; weaving with short floats; and using a tight weave or close loops.
However, styling requirements frequently prevent sufficient use of the foregoing techniques to reduce pilling propensity. In such cases, resource has been made to various finishing techniques: in particular, heatsetting of the fabric or treating the fabric with synthetic resins such as silicic acid preparations (to reduce fiber mobility and migration); cropping and singeing (to remove the fiber ends that project out of the fabric); and treating with special chemicals to reduce the strength of the fibers.
Concerning treatment with special chemicals, it has been stated that: "by applying a carefully controlled treatment with certain chemicals during finishing, e.g., with 3-5% ammonia solution at 125.degree.-130.degree. C., the strength of the polyester fibers can be reduced to such an extent that the resulting fabrics have a low pilling tendency. The treatment can be applied to loose stock, slubbing, yarn or woven or knitted fabrics, depending on the form available and on the method of treatment chosen." However, such a process is not known to be in commercial use, perhaps on accounting of controlling the balance between excessive pilling and loss of fabric strength.
Also, U.S. Pat. No. 3,834,868 includes a detailed discussion of the prior art and describes an invention that is largely relevant to the present invention. In particular, the patent discloses a process in which fine particles of a treating agent containing certain acidic corrosive materials are sprayed onto the surface of the fabric to be treated and the fabric is thereafter heated, to corrode the fluffs on the surface of the fabric, thereby reducing the pilling propensity of the fabric.
While U.S. Pat. No. 3,834,868 itself is apparently silent concerning any use of alkaline saponification agents to reduce the pilling propensity of fabrics containing polyester staple fibers, some such art exists.
For example, I.C.I.'s U.S. Pat. No. 3,287,787 discloses the immersion of polyester fiber in solutions of caustic soda to preferentially stress corrode crimped polyester fiber at the crimp apexes. The patent teaches that the fiber should not have been heatset prior to treatment, and that with such fiber, treatment temperatures "below 110.degree. C., preferably room temperature, up to about 70.degree. C. are preferred" (see U.S. Pat. No. 3,287,787, column 4, line 17-25 and FIG. 3). I.C.I.'s publication "Caustic Treatment of Jersey Fabric Containing Terylene Staple Fibre" (May, 1974) includes the following:
"The basic treatment is 30 minutes in 2 g/l caustic flake at 60.degree. C. It is important to note that such treatment must be carried out before heatsetting, otherwise its efficiency is greatly reduced. The two most convenient points at which to apply the caustic are during either scouring or reduction-clearing, the former usually giving slightly better pilling resistance.
"If applied during scouring, the soda ash component of the scouring bath is replaced by 2 g/l caustic flake, and if applied during reduction-clearing the ammonia component is replaced by 2 g/l caustic flake. Whichever technique is adopted, other conditions will remain unchanged (but see below).
"Increasing the caustic concentration above 2 g/l has no beneficial effect, nor does increasing the time beyond 30 minutes, where the maximum effect is obtained. The factor with the greatest effect on the efficiency of the treatment is temperature. Optimum improvement in pilling resistance is obtained by operating at 90.degree.-100.degree. C., but at this temperature a significant reduction in abrasion resistance is probable. High-temperature treatment should thus be used only with (a) fabrics that are very difficult to control for pilling or (b) those fabrics where very low abrasion levels are acceptable.
"The addition to the caustic scour or reduction-clearing bath of reagents, such as quaternary ammonium compounds, which accelerate caustic attack on polyester will improve pilling almost to total elimination, but the fabric will be completely unacceptable due to extreme loss of abrasion resistance. This procedure is therefore not recommended."
There has also been public use more than one year prior to the filing of this application, by applicant's assignee, of a process involving (i) padding of already dyed fabrics with solutions of 2 percent and 4 percent sodium hydroxide, plus an accelerator; and (ii) heat treating the product at temperatures of 350.degree. F. for 20 seconds. The report concerning this public use concluded "The data attached show that 2% and 4% are better than no caustic and that 4% is worse than 2%." The process also involved singeing the product (to be salvaged by a customer) and even at 2% concentration the overall improvement in pilling propensity was only from 1-2 to 4, as measured by the Random Tumble Pilling Test (described below).
Sodium hydroxide has also been very extensively used in the processing of fabrics containing polyester fibers for purposes other than improving the pilling propensity. However, no art has been found relating to the treatment temperatures of fabrics containing polyester staple fibers in which the following combination of both high concentration of sodium hydroxide and high treatment were used: concentrations of at least 5 percent sodium hydroxide, and temperature of at least 115.degree. C.
For example, U.S. Pat. No. 2,828,528 claims a process involving immersion of a polyester fabric in an aqueous solution of sodium hydroxide until the fabric has lost at least 5 percent in weight; and exemplifies the use of an aqueous solution containing 3% by weight sodium hydroxide at temperatures up to 100.degree. C. The whole process results in a more silk-like product.
Also, U.S. Pat. No. 4,008,044 relates to a process for improving the soil release and wetting properties of a polyester textile material. The process involves, firstly, applying an aqueous solution containing up to 2.5 by weight of sodium hydroxide; and, secondly, then contacting the fabric with saturated steam for at least 10 minutes. This patent also includes a discussion of other prior art concerning the treatment of fabrics with aqueous solutions of sodium hydroxide.
It is also well known that mercerization of fabrics and the fixing of certain dyes involve the use of solutions of sodium hydroxide.
Accordingly, none of the foregoing prior art exemplifies anti-pilling treatment of a fabric containing polyester staple fibers which involves spraying the surface of a fabric with an aqueous solution containing at least 5 percent by weight of an alkaline saponification agent, and then heat treating the sprayed fabric at a temperature within the range 115.degree. C. to 165.degree. C. Further, there are teachings in the art that, individually, show high temperature is undesirable and that high concentration is undesirable.